To view an alternate ending, beat the game without getting the "Game Over" screen after the credits involving the arrival of a certain character. If you get kicked out of the hotel or get sent back to your room, just reload your previous saved game file.
Since the game has a lot of text, and game overs result in quite a lot of lost playtime, use the following hint for red questions. Before prompted to ask red questions, exit out of the screen and save the game. Then, go back in to see every short scene where your character's face and the other will appear on each screen before going back to the red question list. By doing this, if you get a game over, you can go back to the saved game file and just start again at the question list.
On your second playthrough, Louis gives you a "Mysterious Tape" in Chapter 9. Play it and take down what it says. Later, while stuck in the basement, copy the message into the Caesar Shift decoder. The message "A Memento in a Room of Love" will appear. Near the end of the game, talk to every single person before going into Room 220 (Love) to get "Mila’s Treasure."
You'll get to an office in the game where you must insert a specific combination to open a lock. The combination is 13-. (The final number will change from game to game). But it won't work, even if you know the combination, until you pick up the piece of paper lying on the floor near the cabinets in the room.